The word while signals a change in direction, and the second half of the sentence is negative (something is coming “too late”), so you need something in the blank that is positive. In the case of environmental concern, that would be “more of it.” Both burgeon and mushroom fit that meaning.

Disseminate is close, but it has no pair (promulgate means “to make something widely known,” not “to become widely known”), and the concern isn’t spreading out so much as growing.

Hence options burgeon, mushroom are correct!_________________

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